Towards the elimination of chromatography in protein purification: Expressing proteins engineered to purify themselves

The development of affinity-tag fusion technology in the mid-1980s provided a simple and convenient method for the purification of arbitrary recombinant proteins through genetic engineering. This method has recently been enhanced by the introduction of self-cleaving affinity tags based on engineered self-splicing inteins. The ability of these tags to release native product proteins from fusion precursors, in response to simple temperature or pH changes, has increased the appeal of this method for large-scale applications. We have recently developed two simple, cost-effective and scaleable protein purification technologies that exploit the self-cleaving reaction in novel, non-chromatographic contexts. In the first system, small granules of polyhydroxybutyrate (PHB) are produced in the expression host, which then act as an affinity carrier for the subsequently expressed target protein. The protein is tagged with a self-cleaving PHB-binding protein, allowing simple purification of the native product by simple granule washing and tag cleaving. In the second system, the target protein is tagged with a reversibly precipitating, self-cleaving elastin-like protein, allowing it to be separated from contaminants by simple cycles of precipitation and washing. The self-cleaving reaction again delivers a fully native, substantially purified product. Tests on several target proteins for both technologies show high purity and activity at reasonable yields, while initial economic analyses of these methods indicate a potential to dramatically reduce the cost of protein purification. Thus, the introduction of self-cleaving purification tags has enhanced an existing technology while enabling new technologies, and is likely to have a profound effect on the future of bioseparations.